Biology Reference
In-Depth Information
In yeast, MG is produced by the spontaneous breakdown of the glyc-
eraldehyde-3-phosphate metabolite (
Inoue, Maeta, & Nomura, 2011
). By
contrast, in bacteria, MG is mainly synthesized enzymatically from the gly-
colytic intermediate dihydroxyacetone phosphate (DAHP), via the action
of MG synthase, which is allosterically controlled by DHAP and feedback
inhibited by inorganic phosphate (
Masip et al., 2006
). In cyanobacteria,
only the nonmarine strains possess a presumptive MG synthase-encoding
gene (
Table 5.3
), which appeared to be crucial to cell growth in
Synecho-
cystis
PCC 6803 (Narainsamy, Chauvat and Cassier-Chauvat unpublished
results).
GSH plays a major role in cell protection against MG (
Fig. 5.1
), a func-
tion catalysed by the two enzymes glyoxalase I (GlxI) and glyoxalase II
(GlxII), which are ubiquitous (
Inoue et al., 2011
;
Lee et al., 2012
;
Masip
et al., 2006
;
Yadav, Singla-Pareek, & Sopory, 2008
). GlxI (S-D-lactoylglutathi-
one methylglyoxal lyase) converts the spontaneously formed hemithioacetal
adduct between GSH and MG, to S-D-lactoylglutathione. This glutathione
thiolester is then hydrolysed by GlxII (S-2-hydroxyacylglutathione hydro-
lase) to produce the nontoxic D-lactate and regenerate GSH (
Suttisansanee
& Honek, 2011
). Both GlxI and GlxII are single enzymes in
E. coli
(
Masip
et al., 2006
), whereas the yeast
S. cerevisiae
possesses a single GlxI enzyme
and two GlxII enzymes, which are all dispensable for normal growth (
Inoue
et al., 2011
). In bacteria, the metalloenzymes GlxI with shorter amino acid
sequences (∼130 amino acids in length) tend to be Ni
2+
/Co
2+
-activated
(
Mullings, Sukdeo, Suttisansanee, Ran, & Honek, 2012
), while longer GlxI
(∼180 amino acids in length) are likely Zn
2+
-activated enzymes (
Suttisansa-
nee et al., 2011
). In yeast, GlxI contains both Fe and Zn (
Inoue et al., 2011
),
and the expression of its gene is regulated by osmotic stress conditions
to combat the increased production of MG accompanying the increased
synthesis of the glycerol osmolyte (
Inoue et al., 2011
). GlxII is a metallo-
enzyme with binuclear active sites per monomer that can be activated by
various metals, depending on the particular source organism. As isolated,
E. coli
GlxII binds 1.7 mol of Zn per mole of monomeric enzyme, while
other metals were not detected (
Suttisansanee & Honek, 2011
).
Besides the GlxI/GlxII system, various organisms have other routes to
detoxify MG. In
E. coli
, the heat shock protein Hsp31, renamed as glyox-
lase III, directly converts MG to D-lactate without the need for the GSH
cosubstrate, unlike the GlxI/GlxII system (
Subedi, Choi, Kim, Min, & Park,
2011
;
Suttisansanee & Honek, 2011
). In addition, MG can be reduced to
L-lactaldehyde by the NADPH-dependent MG reductase, also present in
S.
cerevisiae
(
Inoue et al., 2011
). Furthermore, MG can be converted to acetol
